Within the field of application of the present invention it is possible to stimulate biological tissue such as the brain, the peripheral nervous system, muscles and the heart of a subject by inducing an electric field in the tissue. In terms of magnetic stimulation, the induction of the abovementioned electric field is accomplished by means of a changing magnetic field. It will be appreciated that such an electric field generates an electric current in the conducting tissue which stimulates the tissue. Various different types of methods and apparatus for magnetic stimulation are known in the industry.
The stimulation of a brain by a changing magnetic field is known as transcranial magnetic stimulation (TMS). Transcranial magnetic stimulation is a non-invasive method used to depolarize or hyperpolarize the neurons of a subject's brain. TMS uses electromagnetic induction to induce weak electric currents using a rapidly changing magnetic field; this can cause activity in specific or general parts of the brain with minimal discomfort, allowing the functioning and interconnections of the brain to be studied. A variant of TMS, repetitive transcranial magnetic stimulation (rTMS), has been tested as a treatment tool for various neurological and psychiatric disorders including migraines, strokes, Parkinson's disease, dystonia, tinnitus, depression and auditory hallucinations.
It is known in the industry, that the locations in the brain of a subject which are stimulated are not necessarily those locations of the brain which have received the most cumulative electromagnetic field (EF) exposure. If it is assumed that treatment efficiency correlates with localized cumulative EF exposure, it becomes useful to integrate exposure over time and hit locations, and then visualize the results of the treatment in an intuitive way that provides a more complete picture of the treatment than simply pinpointing the locations of the stimuli.
Conventionally, the determination of the cumulative effects of application of transcranial magnetic stimulation to the brain of a subject is represented through a linear model where the properties of transcranial magnetic stimulation are transformed into dose elements which can be implemented over time. This method is explained in Finnish patent no. FI114613B, which was continued in the U.S. and issued as U.S. Pat. No. 6,849,040, and which is herein incorporated by reference. It is to be appreciated that an issue with said patent is that the effects of decay, where the dose of transcranial magnetic stimulation is applied over time, are not addressed.
In addition, Finnish patent no. FI114613B provides a method of calculating an effective dose comprising integrating cumulative and effective dose applications over the duration of a magnetic stimulation so as to obtain a cumulative result. However, it will be appreciated that an issue with this method is that a threshold value is provided for the magnitude of the stimulus and although a multiplication factor is included, the magnitude of the multiplication factor is dependent on the frequency of the application of the stimulus.